The present invention relates to a process for controlling process parameters in the production of stretched foils in order to make the orientation condition of molecule chains in the foils uniform. The present invention also relates to an apparatus for carrying out the process.
In the production of films from thermoplastic materials, the polymer granulate is formed into a molten film by means of an extruder and a flat-sheet die. The molten film hardens on contact with cooling rollers to produce an intial film, which is then stretched, either simultaneously or in stages, by means of stretching devices. This results in an orientation of the molecule chains in the foil material, which is essentially created by the stretching process. Usually, longitudinal stretching in the machine direction takes place in the first instance. Subsequently, lateral stretching takes place in the cross direction or the direction transverse to the machine direction. The lateral stretching is, in general, carried out in a frame. After completion of the lateral stretching, the foil band is subjected to a temperature treatment, which results in thermal fixing of the orientation of the molecule chains in the foil, thus maintaining the form of the foil material. As a result of the stretching, a partially crystalline and biaxially oriented foil is produced. The orientation within the foil can be described by the index ellipsoid, which is set by the refractive indices n1, n2 and n3. Depending upon the existing orientations of the molecule chains in the foil, different refractive indices exist in three mutually perpendicular axial directions.
From German Offenlegungsschrift No. 3,001,881, a process is known for the analysis of the surface quality of flexible materials, more particularly of foil webs. According to the process, the surface quality is analyzed by interference fringes which are produced by means of a light beam which is directed obliquely onto the surface of a test object to be investigated. Part of the light beam is reflected at the surface and another part is reflected at a reference surface. By superposition of the two partial beams, an interference field is created, which is equivalent to the test object surface profile set against the reference surface. The interference field is evaluated in order to determine the spread of the surface profile in its length and height with reference to the spacings of the interference lines and the wavelength of the incident light. The test object to be investigated is set on the reference surface under predetermined tensile stress. The light beam directed onto the surface to be investigated is monochromatic. By this process, unevenness in the surface of foil webs can easily be detected, it being possible to determine the height of each unevenness by counting the number of interference rings.
In German Offenlegungsschrift No. 3,243,719, a laser-optic arrangement is described for the production of scattered light, shadow, or reflection signals from individual particles which flow in a flow cell and which are registered by a photodetector by means of an appropriate optical imaging system as temporal tension/current fluctuations. According to the laser-optic arrangement, the temporally fluctuating scattered light signals of flowing particles and/or particle aggregates are detected and evaluated. This makes it possible to carry out a significant determination of the degree of dispersion in systems, corrected for practical conditions, within measurement times smaller than 5 seconds, and to use same as a standard to increase the performance of dispersing or flocculating systems.
In biaxially oriented foils, it is generally the case that--depending upon the process parameters--different orientation conditions of the molecule chains exist in the longitudinal and transverse direction. In particular, in biaxially stretched polyester foils, conditions of optical anisotropy are observed in the longitudinal and transverse direction. These differing conditions are caused by a deviation of the principal orientation direction of the molecule chains in dependence upon the foil width. Thus, for example, in the transverse direction of the foil, the parts at the edge of the foil exhibit different orientation conditions from the center of the foil. The deviation in dependence upon the foil width is designated as BOWING or a BOW effect, which--related to the foil width--causes the range of physical properties of the foil to be nonhomogeneous, which can, in some cases, be associated with a reduced mechanical quality of the foil.